System and method for ultrasonic harmonic imaging for therapy guidance and monitoring

ABSTRACT

An ultrasound system is provided which includes a therapy ultrasound transducer and a diagnostic ultrasound transducer and operates in accordance with a method to direct the application of the therapy ultrasound. The method includes operating the diagnostic ultrasound transducer to acquire a first ultrasound image; simultaneously operating the diagnostic ultrasound transducer and therapy ultrasound transducer for a second interval to acquire a second ultrasound image; and determining a difference in the first and second images indicative of the pattern of the therapy ultrasound transducer signal. The difference in the images, which result from enhanced non-linearities and propagation distortions induced by the high intensity therapy ultrasound, can be obtained by subtracting the two images. A method is also provided for monitoring the progress of high intensity therapy ultrasound which evaluates transient changes due to in-situ heating as well as permanent changes which result from cell microstructure alteration.

PRIORITY AND RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 10/773,132 entitled SYSTEM AND METHOD FORULTRASONIC HARMONIC IMAGING FOR THERAPY GUIDANCE AND MONITORING filedFeb. 5, 2004, now abandoned which is a divisional application of U.S.patent application Ser. No. 10/350,994 now U.S. Pat. No. 6,726,627entitled SYSTEM AND METHOD FOR ULTRASONIC HARMONIC IMAGING FOR THERAPYGUIDANCE AND MONITORING filed Jan. 24, 2003 which is a divisionalapplication of U.S. patent application Ser. No. 09/634,272 now U.S. Pat.No. 6,533,726 entitled SYSTEM AND METHOD FOR ULTRASONIC HARMONIC IMAGINGFOR THERAPY GUIDANCE AND MONITORING filed on Aug. 8, 2000 and of whichclaims the benefit of U.S. Provisional Application Ser. No. 60/147,769,filed on Aug. 9, 1999, all of which are hereby incorporated by referencein their entirety.

FIELD OF THE INVENTION

The present invention relates generally to ultrasonic imaging and moreparticularly relates to the use of harmonic imaging to guide and monitorthe application of therapeutic ultrasound.

BACKGROUND OF THE INVENTION

It is known in the art of medical imaging and therapy that ultrasonicenergy can be used for both diagnostic purposes and therapeuticpurposes. For example, high-intensity focused ultrasound (HIFU) beamscan be used to treat tumors by causing local focal temperature increasesthat cause cell necrosis. In using HIFU, the location of the focusedultrasound beam must be determined to place the beams focal point on thetissue (tumor) which is targeted for therapy. In addition, it isdesirable to sense and monitor changes which are induced by the HIFUbeam within the exposed tissue.

It is known that non-linear propagation occurs in a medium, such astissue, which is exposed to intense ultrasound pressure, such as thatwhich occurs from a HIFU beam. The HIFU beam is a high intensitypressure wave which alternately compresses and relaxes the tissue duringa signal cycle. As a beam propagates, regions of compression can disturblocal propagation speeds and result in regions of increased speeds incompression segments and decreased propagation speeds in rarefactionsegments of the wave. This effect locally increases with increasing peakpressure values and also exhibits a cumulative nature, i.e., becomingmore prominent as an intense beam propagates further into a medium. Thistends to distort the propagating pressure wave and enhancenon-linearities in the echo signal. This results in a generation ofhigher-order harmonics and mixing products in a propagating ultrasoundsignal. This process is altered, however, by attenuation losses intissue, which typically increase with increasing frequency.

The use of harmonic imaging in diagnostic imaging is also known in theprior art. However, such systems have been generally used to imagenon-linear scattering from small, gas-filled contrast agent particles.An example of this can be found in applicants' copending applicationSer. No. 09/318,882, filed on May 26, 1999 and entitled, “UltrasonicSystems and Methods for Fluid Perfusion and Flow Rate Measurement,”which is hereby incorporated by reference. There are also reports thatdifferent tissues exhibit different non-linear properties that can beobserved in second-harmonic images if the peak pressures of the launchedpulse are sufficiently large.

Although harmonic imaging techniques have been evaluated and thenon-linear properties of tissues have been observed in the past, theseeffects have not been used in an advantageous matter to guide andmonitor the progress of therapeutic ultrasound.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a system and methodfor aiming a therapy ultrasound beam.

It is a further object of the present invention to provide a system andmethod for monitoring the application of a therapy ultrasound beam usingharmonic imaging.

It is another object of the present invention to provide a system andmethod for detecting thermal lesions resulting from the application oftherapeutic ultrasound and determining the position of such lesionsusing ultrasound harmonic imaging.

A present method is used for aiming or directing a focused ultrasonictherapy beam. The method begins by acquiring a first image scan using afirst frequency ultrasound signal. A second image scan is then acquiredusing the first frequency ultrasound signal in the presence of a therapybeam ultrasound signal. Difference properties in the first and secondimage scans are then identified to determine where the focusedultrasonic therapy beam is currently directed. The difference propertiescan be obtained by generating a difference image from the first imageand second image scans. The difference image can then be superimposed onthe first image scan and displayed to illustrate the presence of thefocused ultrasonic therapy beam.

Preferably, the method described is used in conjunction with anultrasound system which includes a diagnostic ultrasound transducer anda high intensity focused ultrasound therapy transducer which arearranged in a collinear fashion. It is also preferred that the imagescans take the form of non-linearity imaging scans, such as harmonicimaging or pulse inversion imaging.

A further method is provided for operating an ultrasound therapy systemhaving a therapy ultrasound transducer and a diagnostic ultrasoundtransducer to direct the application of the therapy ultrasound. Themethod includes operating the diagnostic ultrasound transducer for afirst interval to acquire a first ultrasound image scan; simultaneouslyoperating the diagnostic ultrasound transducer and therapy ultrasoundtransducer for a second interval to acquire a second ultrasound imagescan; and determining a difference in the first and second image scansindicative of the pattern of the therapy ultrasound transducer signal.

A method of monitoring ultrasound therapy is also provided. This methodbegins by acquiring a first ultrasound image (baseline) of a regionsubjected to therapy. High intensity ultrasound is then applied to theregion for a first time period. A second ultrasound image of the regionis then acquired after the first time period. An aggregate inducedeffect can be determined based on the first and second image scans. Thehigh intensity ultrasound can then be discontinued for a second timeperiod and a third ultrasound image is then acquired. Transient changesdue to in-situ heating in the region and permanent changes due toalteration in tissue microstructure in the region can then bedetermined.

Generally, the first time period is selected to be long enough such thatthe applied high intensity ultrasound has a therapeutic effect. Thesecond time period is selected to allow cooling of the region undergoinghigh intensity ultrasound therapy.

BRIEF DESCRIPTION OF THE DRAWING

Further objects, features and advantages of the invention will becomeapparent from the following detailed description taken in conjunctionwith the accompanying figures showing illustrative embodiments of theinvention, in which

FIG. 1 is a block diagram of an ultrasound therapy system having acollinear therapy ultrasound transducer and diagnostic imagingultrasound transducer;

FIG. 2 is a flow chart illustrating a method of operating the system ofFIG. 1 to aim a therapy transducer to a targeted region; and

FIG. 3 is a flow chart illustrating a method of monitoring theapplication of ultrasound therapy by detecting both transient andpermanent changes in a region undergoing ultrasound therapy.

Throughout the figures, the same reference numerals and characters,unless otherwise stated, are used to denote like features, elements,components or portions of the illustrated embodiments. Moreover, whilethe subject invention will now be described in detail with reference tothe figures, it is done so in connection with the illustrativeembodiments. It is intended that changes and modifications can be madeto the described embodiments without departing from the true scope andspirit of the subject invention as defined by the appended claims.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The systems and methods described herein advantageously use signalprocessing techniques to enhance the detection of non-linear propertiesof regions undergoing ultrasound diagnostics or therapy. The termnon-linearity imaging is used to broadly refer such techniques, whichinclude, without limitation, harmonic imaging and pulse inversionimaging and the like. Throughout the disclosure, embodiments aredescribed in the context of harmonic imaging. However, this is onlyintended to be one exemplary manner of performing non-linearity imaging.

FIG. 1 is a simplified block diagram of the present ultrasounddiagnostic and therapy system. The system includes a therapy ultrasoundtransducer 102 for generating a therapy beam 104 which is focused to anapex referred to as the therapy beam focal point 106. The therapy beam104 is generally a high-intensity focused ultrasound (HIFU) beam. Thesystem also includes a diagnostic ultrasound transducer or array 108which can scan a region 108 with an ultrasound signal suitable for imageacquisition. Scanning of this type is known and can be performed byphysically directing the position of a transducer or by altering thepropagation characteristics of the array 108. Preferably, the diagnosticultrasound transducer 108 is arranged in a collinear manner with respectto the therapy ultrasound transducer 102. It is possible for thediagnostic ultrasound transducer 108 to be a receive only device. Insuch a case, the diagnostic ultrasound transducer would receive andprocess harmonic echo signals resulting from the incident signal fromthe therapy ultrasound transducer.

The transducers are coupled to appropriate ultrasoundgenerators/receivers 112, 114 respectively, which are operativelycoupled to a system controller 116. The system controller is alsocoupled to a display unit 118. These system components are well known inthe art of ultrasound imaging and therapy. A General Electric Logiq 700MR ultrasound unit which provides conventional B-mode (brightness mode)images and harmonic ultrasound images is suitable for practicing thepresent invention.

FIG. 2 is a flow chart illustrating a process for aiming the system ofFIG. 1 prior to application of therapeutic ultrasound from the therapyultrasound transducer 102. First, with the therapy ultrasound transducer102 inactive, a cross section image scan is obtained using thediagnostic ultrasound transducer 108 (step 205). This image scan data isstored (step 210). The image scan data can be stored as acquired echodata, post processed image data (pixels) or some intermediate data form.Next, the therapy ultrasound transducer 102 is activated to generate abrief pulse of ultrasound with a large peak-pressure while a signal fromthe diagnostic ultrasound transducer is also applied to acquire a secondimage data scan (step 215). The second image data scan is a non-linearimaging scan, such as harmonic imaging. The pulse from the therapyultrasound transducer 102 is short enough such that no therapeutictissue alteration results, such as heating or necrosis.

The ultrasound excitation from the therapy ultrasound transducer 102enhances the non-linear phenomena in the tissue and when the signalsfrom the therapy ultrasound transducer 102 and diagnostic transducercombine, this results in the generation of enhanced mixing products aswell as second, and higher order, harmonics. If, for example, thediagnostic transducer is operating in a frequency band centered about f1and the therapy transducer is operating in a band centered about f2, thenon-linearities induced in the tissue would result in mixing productsf1+f2, f1−f2 as well as harmonic components n*f1 and n*f2, where n is aninteger greater than 1. This results from the compression of tissueduring the high-pressure portions of the cycle and tissue relaxationduring the low pressure cycles of the therapy ultrasound signal whichalter the propagation properties of the tissue. These effects varydepending on the peak pressure value applied. Thus, the first image andsecond image will differ in regions that experience simultaneouspresence of the therapy beam and diagnostic beam, whose pressures arecoherently additive.

During acquisition of the second image scan, the therapy ultrasoundsignal and the diagnostic ultrasound signal are temporally coexistent.Generally, the therapy pulse is lower in frequency than the diagnosticpulse. Therefore, the coexistence can occur by superimposing the highfrequency diagnostic pulse on a portion of the lower frequency therapyultrasound pulse. Preferably, the high frequency diagnostic signal isphase synchronous with the diagnostic therapy signal such that imagingtakes place at defined portions of the therapy ultrasound signal. As thediagnostic ultrasound signal is scanned across the region where thetherapy ultrasound signal is present, the harmonic image will detect theareas where the signals coincide. Alternatively, the diagnosticultrasound transducer can take the form of a receive only transducerwhich is scanned across the region to detect harmonic components of thetherapy ultrasound signal only.

The difference between the first image scan and second image scan can bedetermined by subtracting the two image scans (step 220). The differenceimage data can be digitally enhanced by the controller 116 usingconventional digital signal processing techniques and then superimposedon the first image to indicate the position of the therapy ultrasoundsignal 104 and focal point 106 (step 225). If the therapy beam is not inthe desired position, the system can be re-aimed and the process can berepeated from step 205. Once the therapy ultrasound signal 104 isproperly aimed, the therapy ultrasound signal 104 can be applied forlonger durations to induce necrosis of the targeted region.

In some cases, the resulting non-linearities from the coincidentaldiagnostic ultrasound signal and therapy ultrasound signal will besufficiently pronounced such that the region occupied by the therapyultrasound signal 104 will be apparent in the image data scan acquiredwhen the two signals are so applied. In this case, the method of aimingthe therapy ultrasound transducer can be performed in a single step ofsimultaneously operating the diagnostic ultrasound transducer 108 andtherapy ultrasound transducer 102 and viewing the resultingnon-linearity image.

The system of FIG. 1 can also be used to monitor the progress of inducedtissue alteration resulting from the application of the therapyultrasound beam 104 by detecting changes in the degree of non-linearityin the tissue, referred to herein as the B/A coefficient ornon-linearity coefficient. Under the influence of the therapy ultrasoundbeam, transient changes in the non-linear properties of the tissue ariseas a result of in-situ temperature changes, and permanent changes in thenon-linear properties of the tissue arise from changes in the tissuemicrostructure. The in-situ temperature changes are transitory, with thetemperature returning to pre-exposure levels as the tissue coolsfollowing therapy exposure. Permanent changes in the tissuemicrostructure result from cell necrosis or other effects, such ascavitation, which result from application of the HIFU beam. Thus,measurement of transient changes can indicate both the degree andspatial extent of induced heating and measurements of permanent changesare indicative of the degree and extent of thermal lesions.

FIG. 3 is a flow chart illustrating a method of determining in-situheating and tissue alteration using progressive image evaluation.Optionally, the process of aiming the therapy beam set forth in FIG. 2can be applied to locate the therapy beam 104 (step 305). The therapyultrasound beam 104 is then activated and baseline image scan data (I1)is acquired using the diagnostic ultrasound array 108 (step 310).Therapy is continued for a time interval (T1) (step 315) and secondimage scan data (I2) is then acquired (step 320). The therapy transducer102 can be deactivated for a second time interval (T2) (step 325). Thepurpose of the second time interval (T2) is to allow the temporaryin-situ heating effect to dissipate. This period will vary depending onthe power of the applied therapy ultrasound, the duration of thetherapy, the nature of the tissue and the depth of the area beingtreated, among other variables. However, a time period of about 5-10seconds for T2 is considered reasonable to allow sufficient coolingunder various conditions. A third image scan data is then acquired (I3)at the end of the second time interval (step 330). Preferably, toenhance the non-linear effect in the tissue, if the first and secondimages were acquired with the therapy ultrasound present, the diagnostictransducer is activated along with a brief excitation from the highpressure therapy transducer 102 during acquisition of the third imagedata.

The aggregate induced effect from the application of the therapyultrasound signal is apparent from the difference between the secondimage scan data and the first image scan data (I2−I1) (step 335). Fromthe difference between the third image and the second image (I3−I2),transient effects due to heating can be determined (step 340). From thedifference between the third image and the first image (I3−I1),permanent changes due to alterations in the tissue microstructure can bedetermined (step 345). Once the transient changes and permanent changesare determined, the decision of whether to initiate another therapy beamsession is made (step 350), and control reverts to step 310 to continueor to step 355 to end therapy. If the decision is to continue, theapplication of the therapy ultrasound can be adjusted, if necessary, toalter the in-situ heating detected via the transient changes.

In determining the differences in the image scan data sets, the data canbe compared and processed in various forms. The data can be compared asraw echo data, as processed image data (pixels) or some intermediatedata processing step.

In the image acquisition operations with respect to FIG. 3, the imageacquisition preferably takes place during a intervals when the therapyultrasound beam is being applied. This is preferred because of theenhanced non-linear effects of having the contemporaneous application ofthe diagnostic ultrasound and therapy ultrasound signals enhance imageacquisition. However, each of the images (I1, I2, I3) can also beacquired with the therapy transducer turned off. In this case, harmonicimaging can still be performed, but with somewhat reduced efficacy.

In the method of FIG. 3, the B/A effects in lesions are measureddirectly. However, the present systems and methods can also be appliedto measure lesions indirectly by using harmonic imaging to evaluate“shadowing” that results from such lesions. Shadowing is a decrease inultrasound echo strength which causes a concomitant darkening incross-section ultrasound images. Shadowing results from a relativelyhigh attenuation coefficient in a region of tissue being subjected toultrasound. The incident ultrasound wave is attenuated when it passesthrough this region and the reflected wave is again attenuated as theecho signal returns to the transducer. Thus, the echo signal from distalsites behind the tissue causing the shadow is significantly reduced. Ingeneral, tissue attenuation coefficients increase with increasingfrequency. This not only reduces the intensity of the reflected echosignal, but also reduces the non-linear phenomenon discussed abovebecause the incident pressure of the ultrasound signal is also reduced.Therefore, shadows will be more pronounced when viewed with harmonicimages formed using high frequency harmonic components of the ultrasoundsignal.

The use of HIFU beam therapy can result in thermal lesions. Thermallesions have an attenuation coefficient which is higher than that ofnormal tissue. Therefore, shadowing will result from these thermallegions which can be enhanced by the use of harmonic imaging. Further,because shadowing will be manifest behind a lesion but not in front ofthe lesion, when non-linear imaging is employed, the shadow willgenerally appear to originate from within the lesion and the lesionlocation can be readily identified. Thus, by using the diagnosticultrasound transducer 108 to capture progressive images of a regionundergoing therapy ultrasound, and evaluating these images for theoccurrence of shadows, the formation of lesions during this process canbe readily observed.

The present systems and methods provide a way of effectively aiming atherapy ultrasound transducer to insure that the HIFU beam is properlydirected to a targeted area. In addition, once properly aimed, thepresent systems and methods also provide an effective tool formonitoring the effects of the application of the HIFU beam duringtherapy.

Although the present invention has been described in connection withspecific exemplary embodiments, it should be understood that variouschanges, substitutions and alterations can be made to the disclosedembodiments without departing from the spirit and scope of the inventionas set forth in the appended claims.

What is claimed is:
 1. A method of monitoring ultrasound therapy appliedby an ultrasound therapy system having a therapy ultrasound transducerand a diagnostic ultrasound transducer comprising: acquiring a firstultrasound image scan of the region using the diagnostic ultrasoundtransducer, said first ultrasound image scan being formed by highfrequency components of first diagnostic ultrasound signals emitted bythe diagnostic ultrasound transducer; applying high intensity ultrasoundto a region for a first time period using the therapy ultrasoundtransducer; acquiring a second ultrasound image scan of the region usingthe diagnostic ultrasound transducer, said second ultrasound image scanbeing formed by high frequency components of second diagnosticultrasound signals emitted by the diagnostic ultrasound transducer;discontinuing the high intensity ultrasound from the therapy ultrasoundtransducer for a second time period; acquiring a third ultrasound imagescan using the diagnostic ultrasound transducer, said third ultrasoundimage scan being formed by high frequency components of third diagnosticultrasound signals emitted by the diagnostic ultrasound transducer;determining transient non-linearity changes in the region, wherein thetransient non-linearity changes are detected by evaluating the highfrequency components of each the third and second diagnostic ultrasoundsignals; and determining permanent non-linearity changes in the region,wherein the permanent non-linearity changes are detected by evaluatingthe high frequency components of each the third and first diagnosticultrasound signals.
 2. The method of monitoring ultrasound therapy ofclaim 1 further comprising determining aggregate induced changes in theregion by evaluating differences in the second and the first ultrasoundimage scans.
 3. The method of monitoring ultrasound therapy of claim 1wherein said first, second and third image scans are non-linearityimaging scans acquired in the presence of the high intensity ultrasound.4. The method of monitoring ultrasound therapy of claim 3 wherein thenonlinearity imaging scans are harmonic imaging scans.
 5. The method ofmonitoring ultrasound therapy of claim 1, wherein said first time periodis selected to effect a therapeutic result from the high intensityultrasound.
 6. The method of monitoring ultrasound therapy of claim 1,wherein said second time period is selected to allow in-situ cooling ofthe region subjected to the high intensity ultrasound.
 7. The method ofmonitoring ultrasound therapy of claim 1, wherein the transient changesare determined by subtracting one of the second image scan and thirdimage scan from the other of the second image scan and third image scan.8. The method of monitoring ultrasound therapy of claim 1, wherein thepermanent changes are determined by subtracting one of the first imagescan and third image scan from the other of the first image scan andthird image scan.